Variable gain amplifier



Dec. l0, 1946. D MULLERI JR 2,412,279

VARIABLE GAIN AMPLIFIER Filed June 28, 1944 W J. Elo/nils?, nac y IN V EN TOR.

Patented Dec. 10, 1946` 2,412,279 VARIABLE GAIN ALIPLIFIER David J. Miller,

Jr., Philadelphia, Pa., assignor,

by mesne assignments, to Philco Corporation,

vani

Application June 2s, 1944, serial No.

Claims.

amplifiers especially adapted for use in television systems. More particularly the invention relates to improvements in a variable fading the A variable gain disclosed and claimed in U. S. Patent No. 2,233,317, granted February 25, 941, to P. J. K assigned to the assignee of the present application. The present invention is an improvement over that of the said patent.

In the said Konkle patent there isgdisclosed a fading amplifier c amplifier in response to the blan functions. While the device is ing the operation of the system.

The principal object of the fore, is to provide an improved fading amplifier.

invention, thereblan out and be apparent from th conjunction with the accompanying drawing, in which:

Fig. 1 channel television transmission the present invention may be applied; a

To facilitate a clear and complete understanding of the invention, reference may rst be In a conventional television system a video or picture signal is obtained from a camera Pa., a corporation oi' Pennsyl- Virt 2 tube and head amplifier, represented at il, which signal is transmitted to a blankl era tube and head amplifier lG'supplies a video signal to a bianking out and fading amplier l1 which is also connected to the composite signal and hence 'the television receivers may tained in synchronism with mitter.

illustrated a preferred form and fading amplifier provided by the present invention, which may be emvoltages of the tubes 24 om ue of a common anode resistor 34,

justed that the amplifica branch of the ampiier including 1o output signal is derived by way pacitor 35. An inductor 36 preferably is interposed between the common anode resist 34 and the source of anode potential 29, in order to minimize the effect of the coupling condenser and the inter-electrode capacities of the tubes.

The value of the resistor on of an;1

28 may be so advideo sillal 'the tubes 23 and 32 is the same as the amplioation of any input signal through the other branch amplifier may be varied by varying the across the common cathode resistor 21. To this end, there is provided a vacuum tube 31 having a cathode circuit which includes the resistor 21. The anode of tube 31 is connected to the sistor 38 which may a capacitor 39. be applied to t grid of the coupling capacitor 4l. The grid circuit of tube 31 includes a grid resistor adjustable contact of a voltage divider or potentiometer 43 which has one terminal connected to ground and the other terminal the negative The blanking out 4| periodically raises the of tube 31 and causes increased circuit, thereby across the resistor 21 and effecting periodic cutoff of tubes 24. Whenever desired, the video signal may be faded out or in by manipulation of the potentiometer 43 which varies the gain of tubes 23 gI-Ollnd by and 24 by varying the current flow through re- Y 4v specific embodiment has been shown,

sistor 21.

The variable gain scribed corresponds substantially to that shown in the aforementioned Konkle patent. Such an ampiiiier, while generally satisfactory, ency to become unbalanced when the fading potentiometer 43 is operated. It is practically imuse as tubes balanced condition for different adjustments of the potentiometer 4 is working into a plate load which is common with tube 32 while tube 23 is working into its own plate load. This diiiiculty is further aggravated by the fact that tube 23 is coupled to the grid of tube 32 moves the D. C. component and causes the signal on the grid of tube 32 to operate about the grid bias level of that tube, whereas tube 24 I foregoing obiection is overcome bias voltage appearing 20 an anode re- 25 42 connected to an 30 23 and 40 through condenser 3| which re- 60 is D. C. coupled to the output circuit and pre- The amplifier may be adjusted, as previously described, for operation setting of the potentiometer 43, and as long as the potentiometer adjustment remains the same, the system will maintain its balanced condition. But when the potentiometer adjustment is varied, e. g. to fade the video signal out, the system tends to become unbalanced for the reasons stated above. 1f the blanking signal were not present, this would be immaterial, but the presence of 75 said control signal 4 the blanking signal tends to tion of the balanced amplifier to prevent. The undesired components tend to distort the video signal during the fading operation, and may even affect the synchronization deleteriously.

' present invention,

by fading out the potentiometer 43 is operated in a signal. 'To this end there is provided an additional tube 44 having a coupling capacitor 45 connected to the source of blanking out signal. The tube 44 has an anode resistor 46 connected to the source of anode voltage and is coupled to tube 31 through condenser 4i. The cathode of tube 44 is connected to ground, and its grid-tocathode circuit is completed by a grid resistor 41 connected to an adjustable contact on a potentiometer 48 which is connected between of bias voltage. It that opposite ends of the potentiometers 4B and 43 are connected to ground and that the dotted line representation between the adjustable contacts thereof indicates a common control for the two potentiometers. The tube 44 preferably is a small triode, such as a 6J5G, whereas the tube 31may be a larger triode, such as a 6AC5-G or a beam power amplifier such as a 6V6. By this arrangement, tube 44 is biased to cut off when potentiometer 43 is operated in a direction to apply a more positive voltage to the grid of tube 31, By careful selection'of the values of the potentiometer resistances, the blankbe caused to fade out before it When opposite direction to fade in the video signal, the blanking signal tiometer 4B, and when the normal setting of potentiometer 43 is reached the balanced condition of the amplifier is restored.

scribing the present invention a certain preferred understood that the invention is not to be limited thereby since such variations are contemplated as may be commensurate with the scope of the appended claims.

I claim: t l. In a television system, a video signal amplifier, means responsive to a control signal for varying the gain of said amplifier, manuallyoperable means for varying the gain of said amplifier from a normal value to substantially zero or vice versa, whereby'the video signal may be faded out or in, and means operable coniointly with said last-named means for preventing the control signal from deleteriously affecting the amplifier during the fading operation.

2. In a television system, a video signal ampliiler, a control signal source, means responsive to said control signal for varying the gain of said amplifler, manually-operable means for varying the gain of said amplifier from a normal value to substantially zero or vice versa, whereby the video signal may be faded out or in, a vacuum tube interposed between said control signal sourcey and said amplifier, and means for reducingthe conductivity of said tube during operation of said manually-operable means to reduce the gain of said amplier.

3. In a television system, a video signal amplifier, a control signal source, means responsive to for varying the gain of said produce undesired output components, whose presence it is -the funccontrol voltage, said voltage being applied to common cathode circuit, a resistor in said comably biasing said rst tube to fade the video signal amplier substantially to zero, manually-operchannels, an amplifier in each of said channels, responsive to the fading operation for preventing the blanking signal from introducing such efond auxiliary vacuum tube interposed between plifier circuit constructed and arranged to transto reduce the gain of said amplifier substantially of said auxiliary vacuum tubes, whereby the video to zero, manually-operable means for variably signal of one channel may be faded out and the biasing said gain control tube so as to fade the video signal out or in, said blanking signal tendgain of said amplifier, and manually-operable nal may be faded out and in. means for varying the gain of said am lier to 7. A blanking out and fading amplifier for use DAVID J. MILLER. JR. 

